Zinc: an essential metal for maintenance of female fertility

Abstract

Zinc is one of the most abundant transition metals and plays an essential part in cells, including oocytes. It has a variety of roles such as catalytic, structural, and regulatory functions in every cell. Its deficiency is associated with over production of reactive oxygen species (ROS). These roles are firmly controlled and alteration from normal levels can be potentially toxic to cell function. For example, in oocytes, zinc accumulates during oocyte growth and is stored in lipoproteins for embryonic development. It can also play a vital role in the completion of meiosis I in oocytes for maturation thereby leading to ovulation failure if not present in sufficient quantities. However, intracellular mechanisms regulated by zinc, pertaining to female reproduction and ovarian function remain unclear. We hypothesize that exposure to toxins such as British Anti-Lewisite (BAL) and 2,3-Dimercapto-1-propanesulfonic acid (DMPS), a zinc scavenger, cyclophosphamide (CTX) and acrolein, both chemotherapy drugs, and styrene, a commonly used insulation material, deteriorate oocyte quality through a mechanism that involves zinc deficiency and overproduction of ROS. We sought to investigate the impact of zinc depletion in mouse oocytes after exposure to increasing concentrations of a series of toxins such as BAL, DMPS, CTX, acrolein and styrene. Metaphase II mice oocytes (n=90) were pretreated with increasing concentrations of toxins (25, 50, and 100 uM) that reach their peak plasma levels. In all cases, the oocytes were stained with 25 uM of Zinquin (a zinc sensitive and cell–permeable fluorescent probe) and incubated for 1 hour at 37°C, 5% CO2 to determine intracellular zinc content. Confocal microscopy was utilized to measure intracellular zinc levels in oocytes exposed to the different concentrations of toxins. Oocytes were examined under confocal microscopy and images using a Zeiss LSM 510 META NLO microscope with emission wavelengths of 480 and 535nm, and 358 and 461 nm, respectively. A decrease in intracellular zinc was noted as compared to the control as function a of increasing concentration of the various toxins as indicated by decreased zinc-mediated blue fluorescence. Florescent intensity was quantitated in terms of corrected total fluorescence (CTCF) and a statistically significant difference was found between the CTCF of the treated groups compared to control (p<0.001). The lowest CTCF was noted for oocytes treated with a zinc scavenger and to a lesser extent to styrene. In all cases exposed oocytes were noted to have an enhancement of ROS production. Collectively, these results demonstrate that the deterioration of oocyte quality is associated with intra-oocyte zinc depletion. Exposure to these toxins is associated with deterioration of oocyte quality, zinc depletion and enhancement of ROS thereby leading to subfertility. Collectively, zinc is vital in female reproduction and therefore exposure to such toxins should be avoided in reproductive age women.